I immediately thought of Keegan Connor Tracy, whom I first discovered on another show that was too good to go so soon, Jake 2.O.
http://imdb.com/name/nm0870535/
She's a wonderful actress, showed a lot of range in a short time on Jake, and has languages and martial arts training. I am assuming the story would have her taking the place of Sam, maybe so he can have a life, or she just wants to, and therefore she would need skills like his - or maybe there are lots of leapers ? Whatever. I like that, unlike some other suggested actors, she has delicate features like Melora does, rather than an angular face - although Melora, sadly, looks more bony now than she did then, her bone structure isn't as heavy and sharp as many. Keegan has dark wavy hair like Melora did in QL, and a certain open, emotional quality that both TV "parents" share - I think that's partly acting, but also to do with looks and personality.
She certainly showed emotion of all kinds on Jake 2.0. It was interesting how she was portrayed first as smart and shy (like Sam), kinda soft, but later we see her angry and even taking charge, believably. Sigh. Good writing, and women who are people. Instead now we have "Chuck", with an unappealing geek guy and all the females as bimbos - sorry, drifting there.
She's a little short, but they can work around that. Genetically speaking, it's not as improbable as some might think; Melora's not that tall, and on a personal note, my sister and I are both much shorter (5-7") than either of our parents, and my other sister is taller than both.